Safe JSON stringify

gistfile1.js

    // A JSON stringifier that handles cycles safely.
    // Usage: JSON.stringify(obj, safeCycles())
    function safeCycles() {
        var seen = [];
        return function(key, val) {
            if (!val || typeof val !== 'object') {
                return val;
            }
            // Watch out for Window host objects that are trickier to handle.
            if (val instanceof Window || seen.indexOf(val) !== -1) {
                return '[Circular]';
            }
            seen.push(val);
            return val;
        };
    }

This is from isaacs/node-bunyan@c2338b1